It was known in the art to produce structural cases with flanges and structural ribs which projected radially from a flat annular wall. The use of flanges and ribs is a way to provide structural resistance by adding a relatively small amount of weight as compared to simply increasing the thickness of the wall. In order to be reproducible and to satisfy shape tolerances, ribs were formed by removing material (e.g. milling pockets), leaving the ribs around a pocket and between pockets.
Milling requires specific machinery and results in a relatively high amount of removed material not being used in the final component. Milling is thus a relatively expensive process. Furthermore, milling has a limited precision concerning wall thickness at the bottom of the pockets in a component such as an annular structural case, and the resulting wall thickness can thus have occurrences of thickness variations imparted by the milling step. To be structurally satisfactory, the wall thickness is designed to a given minimal thickness, to which the milling variation thickness is added to ensure the minimal thickness is always present. The designed wall is thus typically thicker than structurally required. This extra ‘safety thickness’ results in extra weight.
Accordingly, there remains room for improvement in addressing the manufacture of structural cases for aircraft gas turbine engines.